1. Field of the Invention
The present invention relates to a White-Taylor Guest-Host (WT-GH) type liquid crystal display having better threshold voltage characteristics and capable of being easily driven in a multiplex mode.
2. Description of the Prior Art
Guest-Host display devices are generally grouped into those employing polarizers and those not using polarizers. White-Taylor Guest-Host type display devices are representative of the latter group. This type of display device employs a liquid crystal mixture composed of a P-type nematic liquid crystal with a dichroic dye and an optically active material added thereto. The display device utilizes for generating its display both the absorption of light (colored state) due to a twisted orientation of liquid crystal molecules in the absence of an applied electric field (or an electric field less than a threshold electric field) and the transmission of light (non-colored state) due to a homeotropic orientation of liquid crystal molecules in response to an applied electric field (greater than a threshold electric field). FIGS. 1(A), 1(B) and 1(C) illustrate the principles of operation of such a liquid crystal display device. In the absence of an applied electric field, dichroic dye molecules 1 and host liquid crystal molecules 2 are twisted between a pair of glass substrates 3 and 3 as shown in FIG. 1(A). When an electric field is applied by power supply 4 across the transparent electrodes 3, the dichroic dye molecules 1 and the liquid crystal molecules 2 are oriented perpendicularly to the electrodes as shown in FIG. 1(B). Incident light having a wavelength of .lambda. max is substantially absorbed when the molecules are oriented as shown in FIG. 1(A), and is transmitted therethrough when the molecules are oriented as shown in FIG. 1(B). As illustrated in FIG. 1(C), the transmitted light has a spectrum which changes when the applied electric field is the ON and OFF states.
The above display system has widely different characteristics dependent on the type of surface alignment. The surface alignment can be divided mainly into homeotropic and homogeneous alignment types. While the display constrast is higher with the homeotropic alignment, the drive voltage required is lower with the homogeneous alignment. Therefore, the homogeneous alignment is better for a low-voltage multiplex drive.